The subject matter disclosed herein relates to combustion systems for gas turbine engines. Manufacturers and operators of gas turbine engines desire to produce and operate gas turbines that will operate at high efficiency while producing reduced quantities of governmentally regulated combustion constituents. The primary regulated exhaust gas constituents produced by gas turbine engines burning conventional hydrocarbon fuels are oxides of nitrogen (“NOx”), carbon monoxide (“CO”) and unburned hydrocarbons (“HC”). The oxidation of nitrogen in internal combustion engines is dependant upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen is a function of temperature. Controlling the temperature of combustion in the combustion chamber to a desired temperature will assist in controlling the formation of NOx components.
One method of controlling the temperature of the combustion system reaction zone in a turbine engine combustor, to a level that will limit the formation of NOx constituents, is to pre-mix fuel and combustion air to a “lean” mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of the combustor will absorb heat and reduce the temperature of the combustion event.
Operational issues involved with combustors operating with lean pre-mixing of fuel and air involve the presence of combustible mixtures within the pre-mixing sections of the combustor, upstream of the combustor reaction zone. In such cases, combustion may occur within the pre-mixing section due to an effect referred to as “flashback” that may occur when the flame from the combustion zone propagates into the pre-mixing section of the combustor. Additionally, auto ignition may occur when the dwell time and temperature of the air/fuel mixture in the premixing section is sufficient for combustion to be initiated without an igniter. Results of combustion occurring within the premixing zone of the combustor may include degradation of emissions performance of the gas turbine engine and/or overheating of the combustor premixing section and lower than desirable durability.
In addition, the mixture of fuel and air exiting the pre-mixer section and entering the reaction zone of the combustor should be uniform so as to achieve the desired emissions performance. If regions exist in the air/fuel flow field where the concentration of fuel versus air is richer than in other regions, the products of combustion in these rich regions may attain a higher combustion temperature and, as a result, a higher level of NOx. Alternatively, regions in the air/fuel flow field where the concentration of fuel versus air is leaner than in other regions may lead to quenching, with a failure to oxidize hydrocarbons and or carbon monoxide, leading to higher than desired CO and HC emissions levels.
It is therefore desirable to obtain a combustor for a gas turbine engine having features that allow a reduction in the emission of regulated constituents with satisfactory performance and durability.